In LTE Release 13, a study item on latency reduction is under progress. One of the proposed concepts, “Fast Uplink (UL)”, is an enhancement of semi-persistent scheduling (SPS) in which a recurring grant is configured for a user equipment (UE) with a period down to 1 (one) transmission time interval (TTI). If configured for Fast UL, when the UE does not have data to transmit it does not have to send padding messages in the periodic grant. Configuring a Fast UL grant for the UE allows the UE to use the granted resource immediately when it has data without having to send a scheduling request (SR) message. This can provide considerable latency gains.
FIG. 1 depicts a scenario in which a transmission from an UE configured with a Fast UL grant is not received by an enhanced nodeB (eNB). In this example, since the eNB does not know beforehand in which grant the UE will transmit data, and since no padding messages are sent when the grant is not used, there is a risk that the eNB will simply miss an initial transmission, and consequently, the eNB will not send HARQ feedback, e.g. an acknowledgement (ACK) or a negative acknowledgement (NACK). In this case, the UE may interpret no feedback as NACK and retransmit with an increased redundancy version (RV), as depicted in FIG. 1, causing an RV mismatch between the RV value the eNB is expecting to receive and the one actually sent by the UE. In another example (not shown), when the eNB does not send feedback, the UE, which is listening to physical HARQ indicator channel (PHICH), may nevertheless falsely detect an ACK and determine that the transmission was received by the eNB. This scenario will cause a loss of data, at least until RLC retransmission.
FIG. 2 depicts an example of another scenario that results in an RV mismatch. In this example, the eNB misses the transmission from the UE and sends a NACK to the UE. The UE receives the NACK indicating that the eNB did not correctly receive the transmission, and decides to perform retransmission. For the retransmission, the UE will increase the redundancy version (RV) from RV=0 to RV=2, to track the number of retransmissions. However, because the eNB missed the first transmission, it will continue to expect to receive RV=0. Again, this creates an RV mismatch scenario between the eNB and the UE.
While Fast UL reduces the latency for uplink transmissions, it may also be desirable to improve reliability for some applications such as vehicle-to-everything (V2X) and other critical machine-type communication (MTC) use cases in which higher reliability, along with lower latency, is targeted. Hence, both the feedback misinterpretation and the resulting RV mismatch between the UE and the eNB could be a more important problem for the low-latency services.